Santa is a time traveller. Notice that when going back in time, he steals the presents which he delivers going forward. So, what do the elves do?
Just because you can, doesn't mean you do.
Making, delivering, and stealing a lot of toys is hard work. Even if you have (theoretically) infinite time, you may still want some help. In addition, some things you can't do on your own, or you may not have the skill or expertise to make or pull off.
In the case of the time-travelling santa, these elves could have the jobs of:
- Literally and Figuratively Guiding Santas (all of them): The Fat Man must have exceptional memory, or he needs help to remember which presents go where, and more importantly, where the santa/anti-santas pairs have been. Also, the psychological stress of being both Santa, anti-Santa, and the Santa for next year will likely require therapy. Perhaps these elves are really experts in psychology as well. They keep Santa working as he should. The Christmas Cycle Must Continue!
- Hide Santa's Powers: They maintain the image of a single santa, trying to hide his amazing powers. They claim to make all of his toys, only so that people don't wizen up to santa's real method. After all, just the ability to visit every house (undetected) in the world and take/leave things would like to be abused by very many, usually naughty people. This does not include the time shenanigans people could do with such knowledge. We shouldn't also ignore the dangers of simply being such a well-known celebrity. (Just because santa can travel forward/backward in time and teleport/phase through things doesn't mean he is immune to surprises!)
- Combining, gathering, and sorting intel during the year: Yes, I know Santa knows when you're sleeping, he knows when you're awake, he knows when you've been bad or good. (So be good for goodness' sake!) However, there are a lot of people and usually one santa. Even if santa knows this through supernatural means, it would only be natural that he needs support to maintain the lists, remembering who did what, what they want, and so forth. It could be that these elves store and deliver information for santa as he needs it. (Alternatively, the elves are how he knows so much about so many people, putting government intelligence agencies to shame.)
Nothing better than a good coverup the elves are actually intelligent bioengineers we believe there is "one" Santa and there is however clones my friends clones. Big brother is always watching and guess what so is your own personal Santa! Hmmm makes me wonder if the santas in shopping malls are on strike???